There are occasions where copyright management data is embedded in contents data such as music motion pictures or game software, in order to limit illicit copying of the contents data. For example, the SDMI (Secure Digital Music Initiative), installed for developing a unified system for the globally usable copyright protection technique, is now investigating into embedding copyright management data in musical air data with watermark signals, with a view to developing a security technique capable of providing music to the consumers in such a manner as to protect the copyright of the music currently in circulation.
For example, the watermark signals are copyright management data, such as copying permitting data or data specifying the number of times of possible copying for the contents data. When recording the contents data into which these watermark signals have been embedded and which have been encrypted and compressed, a data recording device first deciphers the contents data, decompresses the compressed contents data and subsequently extracts the watermark signals from the contents data. If the extracted watermark signals allow for recording, the data recording device re-compresses the decompressed contents data and encrypts the compressed contents data to record the data on a recording medium.
In recording encrypted and compressed contents data by the above method, the cipher of contents data is deciphered and compressed contents data are decompressed. Watermark signals are then detected. After detecting the watermark signals, contents data are re-compressed and re-encrypted, after which the encrypted compressed contents data are recorded on the recording medium. That is, the above-described method is extremely inefficient because of overlapped processing operations, such as re-encryption or re-compression, before recording the contents data on the recording medium.
With this method, in which signal conversion processing, such as re-compression, is involved, the contents data for recording is deteriorated in sound or picture quality. Specifically, the compression processing performs irreversible processing, such as DCT (discrete cosine transform) or quantization. By repetition of the compression processing, data deterioration occurs to a visually or audibly perceptible level.